1. Field of the Invention
The present invention relates to a radioactive gas monitor which is installed in an exhaust pipe of nuclear facilities and which can measure radioactive concentration from an upper limit in a measurement concentration range of a normal time monitor to an upper limit in a measurement concentration range in response to severe accidents.
2. Description of the Related Art
In addition to a normal time gas monitor based on guidelines relating to measurement on emitted radioactive materials in power generating light water reactor facilities, an emergency-use gas monitor is disposed in an exhaust pipe of nuclear facilities in order to estimate the magnitude of a radioactive material amount emitted to the environment when a loss-of-coolant accident occurs in a nuclear reactor, based on examination guidelines relating to radiation measurement when the accident occurs in the power generating light water reactor facilities. The emergency-use gas monitor is designed so that an upper limit value in a measurement concentration range satisfies 3.7×106Bq/cm3. In addition, a lower limit value in measurement is moderately overlapped with the upper limit value in measurement of the normal time gas monitor. However, in recent years, on the assumption of more severe accidents than the loss-of-coolant accident in the nuclear reactor, a gas monitor for coping with severe accidents whose temperature conditions are more stringent than those of the emergency-use gas monitor in the related art and whose measurement range is considerably expanded to a high concentration measurement side has been required.
As a preceding example in a method of expanding the measurement range to the high concentration measurement side, Patent Document 1 discloses a radioactive gas monitor which includes two sample containers having mutually different capacities, uses the large volume container as a low concentration measurement container, and uses the small volume container as a high concentration measurement container. In this example, the high concentration measurement container, a collimator, the low concentration measurement container, and a radiation detector are arrayed sequentially in this order, inside a shield. Flow is automatically switched over between a first sample container and a second sample container by an electromagnetic valve at a predetermined radiation measurement value.
Patent Document 2 discloses a radioactive concentration measurement apparatus in which a movable collimator having an inner diameter equivalent to that of the radiation detector is arranged between one sample container and the radiation detector. In this example, when the low concentration range is measured, the movable collimator is moved close to the radiation detector so as not to interfere with the measurement, and when the high concentration range is measured, the movable collimator is moved close to the sample container so as to narrow down the number of radioactive rays which are incident on the radiation detector per unit time.
[Patent Document 1] Japanese Patent No. 4453729
[Patent Document 2] Japanese Patent No. 4089522
In the sample container switching method disclosed in Patent Document 1, when flowing is switched over from the low concentration measurement container having the large volume to the high concentration measurement container having the small volume, a purge is performed on the low concentration measurement container arranged between the high concentration measurement container and the radiation detector. Consequently, there is a problem in that no measurement can be carried out during the purge.
In the movable collimator method disclosed in Patent Document 2, it is necessary to increase a moving distance of the movable collimator in order to expand a measurement range, thereby increasing a size of the shield. Furthermore, measurement results are unstable while the movable collimator is moving. Consequently, there is a problem in stability and responsiveness in a high concentration measurement range requiring a quick response. In addition, there is a problem in that a mechanism for moving the movable collimator is complicated, or there is a problem of increasing manufacturing cost since a precise stop position of the movable collimator is needed in order to improve measurement accuracy.
Furthermore, in recent years, in order to cope with severe accidents, a radioactive gas monitor has been required in which the upper limit value in the measurement concentration range satisfies 1×1011Bq/cm3. However, the radioactive gas monitor is unlikely to be applied to the above-described sample container switching method or movable collimator method. That is, in the sample container switching method, it is necessary to allow the high concentration measurement container to have very small dimensions. In the movable collimator method, it is necessary to allow the collimator to have very large dimensions. Consequently, both methods are impractical since the manufacturing is difficult.